1. Field of the Invention
The present invention generally relates to a magnetic resonance (MR) imaging system which utilizes an MR phenomenon to acquire structural information, such as a slice image of an object (particularly, a living subject), and functional information, such as spectroscopy. More particularly, the present invention is directed to an MR imaging system suitable for imaging information of motion at a plurality of portions aligned in one direction, such as the blood speeds in renal veins of left and right kidneys.
2. Description of the Related Art
The MR phenomenon is a phenomenon such that a nucleus, which is placed in a static field and has a non-zero nuclear spin accompanied by a magnetic moment, resonantly absorbs and emits only an electromagnetic wave having a specific frequency. This nucleus resonates at an angular frequency .omega..sub.0 (.omega..sub.0 =2x.nu..sub.0 ; .nu..sub.0 : Larmor frequency) expressed by the following formula: EQU .omega..sub.0 =.gamma.H.sub.0
where .gamma. is a gyromagnetic ratio specific to the type of a nucleus and H.sub.0 is a static field intensity.
In a system utilizing such an MR phenomenon to examine and diagnose, for example, a living subject, an electromagnetic wave, induced by the aforementioned resonance absorption and having the same frequency as the aforementioned specific frequency (resonance frequency), is detected and is subjected to signal processing to thereby acquire diagnosis information, such as a slice image of an object, which reflects information about a nucleus density, a longitudinal relaxation time T.sub.1, a transverse relaxation time T.sub.2, the motion of a tissue like the flow of a humor or a chemical shift, without being invaded.
In collecting diagnosis information by a magnetic resonance, it is possible to excite any portion of an object placed in a static field to cause a magnetic resonance and collect an MR signal. Because of the structural restriction to the MR imaging apparatus and clinical demand of a specific portion to be imaged, however, the existing MR imaging apparatuses excite a limited target portion to cause a magnetic resonance and collect an MR signal resulting from the excitation.
In this case, a specific portion to be imaged is generally a sliced portion (a slice) with a certain thickness, and an MR signal, such as an MR echo signal (hereinafter referred to as echo signal) produced at the slice or an FID (Free Induction Decay) signal, is collected by performing excitation of a magnetic resonance which involves a plurality of encoding steps. An image reconstruction is done using such collected MR data in, for example, a two-dimensional Fourier transformation, to provide the image of the specific slice.
The use of such an MR imaging method can provide the imaging of a blood speed. The present applicant has already proposed, as such an imaging method, a method of acquiring blood speed information and tomographic image information together to provide a tomographic image including a blood speed image. This method is the MR imaging method disclosed in the Japanese Patent Application No. 62-204447 filed Aug. 18, 1987 and laid open as the Published Unexamined Japanese Patent Application No. 64-47912. According to this method, a first slice normal to a blood vessel is excited and imaging of a second slice including this blood vessel is executed upon elapse of a predetermined time. In this case, the second slice is excited to cause a magnetic resonance, with the nuclear-spin originated magnetization of the first slice being saturated. When the second slice is excited, the first slice hardly produces an MR signal, so that the image of the second slice shows a portion belonging to the first slice as a blank portion. The pre-excitation of the first slice to form a blank portion in the image of the second slice is called "tagging." With the use of the image of the second slice, it is possible to know the distance blood travels in the aforementioned predetermined time from the positional relation between a blank portion (or a tagged portion) of a blood vessel portion and a blank portion of the other non-moving portion. Based on the distance L over which the blood has moved and the difference in excitation time between the first and second slices, i.e., the moving time T, the blood speed, L/T, can be computed. Information about the speed of a cerebrospinal fluid (CSF) can also be imaged using a similar method.
The conventional method can provide the above image through a sequence of the aforementioned imaging procedures even if a target portion include a plurality of blood vessels as long as they are parallel to one another. If the target portion include blood vessels not running in parallel, such as renal veins those of which from the left kidney and those from the right kidney join, and moreover, if it is significant to compare information on the left kidney and that on the right kidney, however, the conventional method should repeat the mentioned sequence of imaging procedures, one for the blood vessels from the left kidney and the other for those from the right kidney.
This involves a large time lag between the two sequences of imaging procedures, so that the resultant two pieces of information, which should be compared with each other at the same point of time, inevitably reflect the large time difference and cannot be compared at high accuracy. The execution of a sequence of imaging procedures twice takes a considerable amount of time for the imaging, thus impairing the imaging efficiency.